Pharmacology
CNS Pharmacology
Central nervous system drugs for MBBS and NEET-PG: benzodiazepines, antiepileptics, antidepressants, antipsychotics and opioids, mapped to NMC codes.
MedNext Academy | 3 min read
CNS Pharmacology
Central nervous system drugs for MBBS and NEET-PG: benzodiazepines, antiepileptics, antidepressants, antipsychotics and opioids, mapped to NMC codes.
This chapter covers the drugs acting on the central nervous system, including sedative-hypnotics, antiepileptics, antiparkinsonian agents, antidepressants, antipsychotics and opioid analgesics. It links neurotransmitter mechanisms to the therapeutics of anxiety, epilepsy, mood disorders, psychosis and pain.
High-yield: CNS Pharmacology
- Benzodiazepines enhance GABA-A action by increasing the frequency of chloride channel opening, giving anxiolytic, sedative and anticonvulsant effects.
- Flumazenil is the specific antagonist that reverses benzodiazepine overdose.
- Barbiturates prolong chloride channel opening and are far more dangerous in overdose than benzodiazepines, with no specific antidote.
- Phenytoin blocks voltage-gated sodium channels, follows zero-order kinetics and causes gum hypertrophy, hirsutism and teratogenicity.
- Sodium valproate is broad-spectrum but hepatotoxic and highly teratogenic, causing neural tube defects.
- Carbamazepine is first-line for trigeminal neuralgia and can cause a serious rash linked to the HLA-B*1502 allele.
- Levodopa with carbidopa treats Parkinson disease; carbidopa blocks peripheral conversion to reduce nausea and cardiac effects.
- Selective serotonin reuptake inhibitors are first-line antidepressants with a safer overdose profile than tricyclics.
- Tricyclic antidepressants cause anticholinergic effects and are dangerous in overdose due to cardiotoxic arrhythmias.
- Lithium is used for bipolar disorder, has a narrow therapeutic index and causes tremor, hypothyroidism and nephrogenic diabetes insipidus.
- Typical antipsychotics block dopamine D2 receptors and cause extrapyramidal effects and hyperprolactinaemia.
- Clozapine is reserved for resistant schizophrenia and requires monitoring for agranulocytosis.
- Morphine acts on mu opioid receptors; overdose causes pinpoint pupils and respiratory depression, reversed by naloxone.
- Neuroleptic malignant syndrome from antipsychotics presents with rigidity, hyperthermia and raised creatine kinase, treated with dantrolene and bromocriptine.
Key CNS drug facts
- **Benzodiazepines:** Increase GABA-A channel opening frequency; overdose reversed by flumazenil.
- **Phenytoin:** Sodium channel blocker; zero-order kinetics; gum hypertrophy and teratogenicity.
- **Lithium:** Bipolar disorder; narrow index; hypothyroidism and nephrogenic diabetes insipidus.
- **Opioids:** Mu agonists; overdose gives pinpoint pupils and respiratory depression; naloxone reverses.
NMC competencies in this chapter
- **PH1.19:** Sedatives, hypnotics and anxiolytics
- **PH1.20:** Antiepileptic drugs
- **PH1.21:** Drugs for Parkinsonism and other movement disorders
- **PH1.22:** Antipsychotics, antidepressants and mood stabilisers
- **PH1.23:** Opioid analgesics and their antagonists
Frequently Asked Questions
How do benzodiazepines differ from barbiturates?
Benzodiazepines increase the frequency of GABA channel opening and have a ceiling effect with a specific antidote, while barbiturates prolong opening and are lethal in overdose.
Why is carbidopa combined with levodopa?
Carbidopa blocks the peripheral conversion of levodopa to dopamine, reducing nausea and cardiovascular effects and allowing more drug to reach the brain.
Why is lithium monitored closely?
It has a narrow therapeutic index, so small rises in level cause tremor, confusion and toxicity, and it affects the thyroid and kidney over time.
What reverses opioid overdose?
Naloxone, a competitive opioid antagonist, reverses respiratory depression, but it is short-acting so repeat dosing may be needed with long-acting opioids.
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